The present disclosure relates to a phase gradient nanocomposite window fabrication method and a method of fabricating durable optical windows.
Optical windows used in aircraft and high-speed missiles must meet very aggressive requirements on flexure strength, impact durability and optical transparency. Often these constraints are in conflict such that environmentally rugged windows lack sufficient transparency or spectral bandwidth for future generation optical search and track applications.
Optical windows can be produced by various processes that include, but are not limited to single crystal growth, chemical vapor deposition (CVD) and nanocomposite sintering. Nanocomposites, in particular, are very attractive materials for use in windows because they can combine multiple materials (or phases) to produce a window that is stronger than the windows produced from either phase alone. Nanocomposite-based windows are generally formed using a powder process which allows very large and curved window shapes to be produced in nearly finished shape. This is called near net shaping and in theory could be used to form windows of any desired shape with minimal waste of materials. Other window materials that employ single crystal growth or CVD require that window fabrication start from a large block of material that is sculpted to produce the desired window topology. This is extremely expensive and time consuming as well as being wasteful in terms of material usage.
When compared to the single phase (e.g., CVD) materials, the nanocomposites have certain disadvantages that weigh against their use despite the benefits of near net shaping. For example, optical transparency of nanocomposite windows may suffer from increased optical absorption and scattering that is introduced by the added material or phase. That is, near net shape sintering of zinc sulfide (ZnS), for example, produces a window that is mechanically weak and not normally usable in airborne applications but, while the second phase of material that is added prevents large grain growth during sintering (a key to maintaining hardness), the added hardening agent introduces scattering and absorption effects. This occurs, in particular, in windows formed of zinc sulfide and in windows including an additional second sulfide phase. Here, while mechanical strength of the window may be dramatically enhanced relative to pure sintered ZnS, the presence of the second sulfide phase can cause strong optical absorption of radiation in a long wave infrared (LWIR) spectral band.
In addition, while certain coatings and electromechanical interference (EMI) treatment layers can be applied to certain windows, such coatings tend to be insufficiently durable. Meanwhile, although mechanical shutters can be used to protect windows in some cases, shutters are not feasible in all cases.